An overdrive unit is a device which has been commonly used in automotive vehicles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque.
Early manual automotive transmissions were limited to three or sometimes four speeds, with only the more sophisticated having five speeds. This left an unfulfilled need for a higher gear ratio for highway cruising, which was filled by the addition of separate overdrive units (auxiliary or bolt-on), in order to simulate an extra gear.
It is today a desire to achieve even more fuel efficient automotive vehicles and even though manual automotive transmissions today sometimes will have more than 5 speeds, the engine speed will sometimes be too high for optimum CO2 driving at the highest gear.
However, with manual automotive transmissions (gearboxes) it is difficult to add more than 6 speeds, because of the increased effort it means for the driver to manually change gears more frequently. Furthermore, it will be difficult for the driver to decide when to change gears for optimum CO2 driving.
Also, the addition of more gears will normally increase the overall length of the transmission, while the addition of gears in a base transmission would normally require substantial redesign of the vehicle due to the increased space requirements.